Conventional methods of injecting steam into reservoir formations are restricted to single and, in some cases, multiple steam injection apparatus deployed along the length of a downhole steam injection line. The conventional steam injection apparatus, as deployed, releases steam mass flows at a rate causing steam to be injected at random along the length of the reservoir steam injection line. The random injection flows result in different steam mass flow injection rates at each point of injection long the injection line and therefore creates an uneven steam distribution in the reservoir. Uneven distribution of steam in the reservoir reduces the efficiency of bitumen extraction.
Further, conventional steam injection apparatus are typically large in design, and require drilled completions to be large in diameter to permit installation of the apparatus. Conventional steam injection apparatus typically suffer, over time and use, from ingress of sand and debris causing clogging and diminishing the operation of the conventional apparatus.
U.S. Pat. No. 5,141,054 to Almeddine et al. teaches supplying steam to a reservoir using a closed-end tubing or liner in a wellbore, the liner having a plurality of spaced apart perforations bored therealong. Almeddine et al. use a number of perforations of controlled size which act as chokes operating under critical or sonic flow conditions which is dependent upon injection pressure only. Almeddine et al. avoid the use of subsonic flow so as to avoid introducing discharge pressure as a variable in the design of the issuing steam rate. A uniform distribution of steam is purported to be achieved throughout the length of the wellbore by controlled steam distribution through ascertained numbers of perforations in the tubing which are sized and spaced specific to the wellbore so as to achieve said critical flow conditions.
U.S. Pat. No. 6,158,510 to Bacon et al. teaches a relatively large diameter, single tubing string used for both steam injection and production. Spaced apart orifices, all of the same size, bored in a base pipe, are used to purportedly deliver steam uniformly to the reservoir at sonic flows. A wire-wrap screen is formed circumferentially about the base pipe to act as a filter for produced fluids flowing back to the perforated base pipe. The pressure drop across the orifices which governs the maximum steam injection rate achievable through an orifice is affected by the number and size of orifices available as well as the diameter of the base pipe. In a SAGD operation, a separate production well is utilized and the number of orifices in the steam injection liner are constrained such that the pressure drop through the orifices is larger than the pressure drop through either the wire-wrap sections or along the liner itself.
Apparatus such as that taught by Almeddine et al. and Bacon et al. are configured to provide critical or sonic flows. Typically, very large quantities or high pressures of steam or alternatively very small steam injection openings are required to maintain such critical flows of steam from all of the openings along the apparatus. Applicant believes that it is difficult to provide sufficient steam from surface to meet critical flow demands at each opening along the steam injection apparatus. Further, Applicant believes that if openings are made sufficiently small to ensure said critical flow at each of the openings, the amount of heat transfer to the formation may be less than effective for a SAGD operation.
Further, in conventional apparatus, the openings or orifices are typically bored through the tubing or liner. Should changes to the size of the openings be required, the apparatus, specifically configured for a particular situation, cannot readily be reused in other formations, particularly in those which may require smaller openings for lesser steam injection rates.
There is interest in the industry in steam injection apparatus which permits smaller diameter tubing for use in conventional wellbores while using relatively low steam pressures, which reliably results in an equal mass distribution of steam at all points of injection along the injection apparatus which resists plugging as a result of sand ingress from the formation and which has means for delivery of steam which can be re-sized for reuse from formation to formation.